Effect of mutations in the -10 region of the phoE promoter in Escherichia coli on regulation of gene expression.

The phoE promoter region in Escherichia coli contains a -10 region, typical of sigma 70-dependent promoters and, instead of a normal -35 region, a so-called pho box, to which the transcriptional activator phospho-PhoB binds under low phosphate conditions. A second pho box is present upstream of the first one and is required for full expression of phoE during phosphate starvation. To determine whether the lack of expression under high phosphate conditions is due solely to the absence of a genuine -35 box, the -10 region was further optimized towards the consensus -10 sequence and promoter activity was measured using alkaline phosphatase as a reporter. The mutations resulted in a drastic increment in the basal level of expression under high phosphate conditions, indicating that the deviations from consensus in the -10 region also play a role in determining the poor expression of the wild-type promoter under these conditions. The expression under high phosphate conditions was partly dependent on the presence of the phoB gene, showing that a small amount of active PhoB must be present under these circumstances. During phosphate starvation, the activity of the mutant promoters was further induced. The upstream pho box was not required for full expression from the mutant promoters under these conditions. Apparently, the wild-type phoE promoter is carefully balanced by deviations from the optimal Pribnow box sequence that reduce expression under high phosphate conditions and by the presence of several copies of the pho box, which enhance expression under phosphate starvation.